I think the best counter pick againts Yasuo are:
1)Morgana
Now the reason whyis hard because she will Dark Binding you whenever you try to go on her. You can try to dodge it easily by dashing to other enemy minions, then dash on her and cut her down. Her Black_Shield.jpg Black Shield won't really have any use towards you except your whirlwind to prevent her getting knocked up; which means you can't use yourLast_Breath.png Last Breath towards her.
Once Morgana is 6 she can just use her -Soul_Shackles.jpg Soul Shackles on you when you try to go on her. It's a matter of just outplaying her, jungle ganks will help you a lot. Early game she will just push the lane so you might have to roam top or bottom and try to secure some kills.
2)Orianna
Her auto attacks from Clockwork Windup will hurt you a lot, since she also has her Command-Protect.jpg Command: Protect, so she will be pretty hard to face against early game. Try to even the trade when she tries to be aggressive towards you and auto attacks you. You'll need a early jungle gank to stay ahead or you'll be behind the whole time. If you happen to die early try to roam other lanes.
Once Orianna is 6 she'll probably use her Command-Shockwave.jpg Command: Shockwave on herself when you try to push her up and use your Last Breath towards her.
3)Swain
Iis hard because he is a tank AP caster. Early game he can annoy you with his Decrepify +Auto attacks which will do a lot to you. Try to play safe and have your jungler help you gank him so you can try to be ahead.
Once Swain reaches 6 he will be a lot more tanky due to his Ravenous_Flock.jpg Ravenous Flock. Once he gets he will be hard to kill for you if you don't snowball him early. When you dash to him he will try to use his Nevermove on you. You can dash to another enemy minion to avoid it and dash back onto him and cut him down. If you get caught from his Nevermove just prepare to take a lot of damage and be forced to play passively.
4) Syndra
is very strong against Yasuo, not because it's Syndra but because if she knows how to land her stuns and knows how to play the champion you'll have a hard time. If you try to dash towards her she will just Dark Scatter the Weak you and auto attack + Dark Sphere for damage and run away. She will harass you with her Dark Sphere's early game and auto attacks which will force you to play defensive. You'll need a jungle gank early game or you'll be behind. Most AP champs when facing against you will probably rush.
Once Syndra hits 6 she will have a lot more burst from her Unleashed Power. Try not to go aggressive towards her when you're around 60%. Her burst + ignire can kill you off pretty easily, so just be wary of that. If you can bait her Scatter the Weak and dodge it by dashing to another minion then dash back towards her you can try to cut her down since she won't have anymore escapes other than her .